The Fixed Chataat [sin]-offering
Requires Beit HaMikdash
VaYikra 4:27 If a person should unwittingly sin [and he is] one of the common people, by transgressing one of the Mitzvot of Hashem that should not be done and he incurs guilt,
We are commanded that whenever an individual has sinned unintentionally he must offer a Chataat-offering.
Sins which when committed incur the penalty of a Chataat-offering are those for which the penalty is extinction when they are committed willfully, provided that the sin involves violation of a Negative Mitzvah, and that there is some act connected with it. Thus, the Pesach-offering and Circumcision, being Postive Mitzvot, and entailing extinction if willfully transgressed do not involve a fixed Chataat-offering if transgressed unintentionally. Blaspheming the Name, invoivling mere utterance and connected with no act, entails extinction if willfully committed, but does not entail a fixed Chataat-offering if committed unintentionally.
In the Mishneh Torah RaMBaM enumerates forty-three sins which if committed unintentionally entail a fixed Chataat-offering (Mishneh Torah, Korbanot, Hilchot Shegagot 1, 4)
Repentance was also the essential prerequisite for the atonement of all Yisrael on Yom Kippur (see Positive Mitzvah 49)